1. Field of the Invention
The present invention relates generally to communication networks, and more particularly to global multi-satellite communication networks.
2. Related Art
Multi-satellite communication networks have been proposed to address the explosive growth (both existing and anticipated) in global multi-media and personal communication services (PCS), in both the government sector and the private sector. To be effective, a satellite network must provide continuous, error-free, and uninterrupted high-throughput transmission even if one or more satellites in the network fail. This is the case, since it is very expensive to repair a faulty satellite, due to "astronomical" accessing costs. Consequently, fault tolerance, disaster avoidance, and network survivability are important characteristics of any satellite network architecture.
Communication satellites have been in use for a number of years in both the military and private sectors. To date, a number of different satellite systems exist and more are planned.
A distinction should be made between satellite systems and satellite networks (i.e., networks of satellites). FIG. 1 illustrates an example satellite system 101. Generally, a satellite system 101 may consist of one or more satellites 104, and has a plurality of earth stations 106 (located on the surface of the earth 102). Each of these earth stations 106 "see" one or two satellites 104 in space. The satellites 104 are signal regenerators. Information is sent through a satellite system 101 as follows. A signal is uplinked from an earth station 106A. This signal is received by a satellite 104B, which downlinks the signal to another earth station 106B which is usually located thousands of miles away from earth station 106A. By placing a number of satellites 104 in space and around the globe, it becomes possible to send a signal to locations that are very remote from the originating earth station (for example, on the other side of the globe), by hopping up and down between terrestrial stations 106 and satellites 104.
Satellite systems have their advantages and disadvantages. Their disadvantages include transmission delays, echoes, multiple atmospheric signal distortions, traffic capacity limitations, fault intolerance, and potential jamming. Note that, in satellite systems, no inter-satellite communication takes place.
To increase channel (and traffic) capacity, and to overcome potential jamming, computer-based satellites in satellite systems have been deployed with sophisticated multibeam antenna technology. Multibeam antennas make it possible to have multiple fixed spot beams as well as scanning beams and at different frequencies which when under computer control, enable the satellite to autonomously set up data channels at different data rates, and via different antenna beams.
A different satellite system architecture to address traffic capacity, fault tolerance and jamming utilizes a cluster of sophisticated satellites, instead of a single satellite. As an example, LOOPUS is a satellite system with 9 satellites in highly inclined elliptical orbits planned in accordance with a Federal Republic of Germany's industry initiative. This system is planned to utilize beams operating in the frequency range of 14/11 Ghz for the feeder and 14/12 Ghz for mobile links where each satellite will be able to switch 2,000 32 Kbps (kilobits per second) rate channels, reconfigurable to 4,000 or 6,000 channels per region. Here also, satellites in the cluster do not communicate with each other.
A satellite network is a recently developed satellite communication architecture. FIG. 2 illustrates an example satellite network 202, where a plurality of geostationary satellites 206 are positioned in space and around the globe 204. In addition to uplink and downlink capability, each satellite 206 in the network 202 is able to communicate with each other over inter-satellite links (ISL) 210. According to this architecture, an earth station 208A uplinks a signal, which is received by a satellite 206B. The signal travels from satellite to satellite via the inter-satellite links 210 until it reaches its destination.
Three different approaches are typically discussed for implementing satellite networks. One approach involves the use of low orbiting satellites. Low orbiting satellites are easier to launch and have relatively shorter delays, but require significant terrestrial tracking, particularly inter-satellite tracking, and have relatively shorter lives. The second approach involves the use of geostationary satellites. Geostationary satellites are costlier to launch and have relatively longer delays but terrestrial and inter-satellite tracking is easier and their lives are relatively longer. The third approach, which combines many advantages of the first two approaches, employs satellites in inclined circular geosynchronous quasi-stationary orbits, such as the "Molniya" and "Tundra" inclined elliptical orbits.
A satellite network (when compared to a satellite system and for long distance communication) introduces less transmission delays and echoes, increases traffic capacity, and is less susceptible to jamming. Consequently, it is more suitable for real-time interactive multi-media services, such as voice, interactive data, interactive video, private communication channels, and PCS, than traditional satellite systems.
Some satellite networks are already planned. A well known planned satellite network is the U.S. government's MILSTAR satellite program, that has FEP (FLTSAT EHF Package) antennas operating in the extremely high frequency (EHF) and ultra high frequency (UHF) range for Up-link/Down-link (UL/DL) and 60 Ghz cross-orbit links for satellite-to-satellite data transfer. Other planned satellite networks include DARPA's LightSat network, the U.S. Air Force's Reserve network, the U.S. Navy's SPINSAT network, and Motorola's Iridium network.
While representing an improvement over traditional satellite systems, these conventional satellite networks have a number of disadvantages. In particular, these satellite networks do not have advanced fault tolerance, disaster avoidance, and network survivability properties. Also, these conventional satellite networks do not form a complete "network in the sky," since they do not represent a global seamless wrap-around grid of inter-communicating satellites.
Thus, what is needed is a satellite network that has superior fault tolerance, disaster avoidance, and network survivability properties, and that forms a complete "network in the sky around the globe."
Before proceeding further, it may be illustrative to briefly describe a particular terrestrial communication network.